The following are some considerations when you are selecting an air temp sensor / RH sensor.
What is your budget?
Lower-cost sensors will not have the resolution or accuracy of high-end sensors. But usually the lower-cost sensors perform great in the field, don’t require calibration (the RH element is field-replaceable), and meet or exceed the accuracy and resolution that are required of them.
What are the resolution and performance specifications?
The truth is that making a relative humidity measurement is tough. Water in its vapor phase is a shifty molecule and completely non-linear in its behavior across temperature. Water’s behavior changes again when the temperature is below its freezing point. Every sensor on the market will have a tough challenge, with a corresponding high uncertainty, when trying to measure a 90% and greater humidity value. A tougher challenge is to measure relative humidity at below-freezing conditions.
What accuracy do you really need?
Go through the ambient air temperature sensor
’s specifications, and check the response time with the filter. Ask questions. Keep in mind that as soon as that sensor is put inside a naturally aspirated, multi-plate shield, the temperature specifications may have just been skewed. In direct sunlight on a warm day with very little wind, the temperature inside the shield could be anywhere from 0.5° to 1.5°C higher than the actual ambient temperature.
RK330-01 Ambient Temperature Humidity & Pressure Sensor with Radiation Shield
How do you want to handle RH drifting?
All capacitive-type RH chips will drift. Over time the RH element will craze (produce a network of surface cracks), and the RH will drift up and out of specification.
Can you interrupt your measurement data collection to send your sensor in for calibration?
You will have to do this for the higher-end sensors. In some cases, you can swap the head of the sensor out in the field with a calibrated one.
Would you rather walk out to your site and replace the RH chip?
You won’t ever be able to do this for high-end sensors. You just can’t get high-end performance out of a field-replaceable chip.
Always check the compatibility of the data logger that you would like to use with the sensor.
What are the environmental conditions?
For example, does the location for the sensor have consistently high humidity? Is there a lot of blowing dust? What about freezing rain, riming, or horizontal snow? It is important to understand the environment where the sensor will be located.
High-humidity locations will require more frequent visits to either swap out the sensor or replace the RH chip, depending on sensor type. Blowing dust brings its own set of concerns. Dust accumulation on a fan-aspirated or multi-plate shield will skew the temperature on bright, sunny days. Dust will clog the filter material. The sensor measurements will be off after a multi-plate or fan-aspirated shield is covered with ice or packed with snow. At this point, the sensor is measuring the environment inside the shield and NOT the outside.
Don’t forget the shield! Even with all the concerns of using a shield with the sensor, the shield still must be used. Outdoor applications require that the sensor be put into some type of shield to protect the sensor from direct solar thermal loading, direct rain/snow, and blowing debris. Use a shield if the sensor is used indoors in extremely dusty environments. The shield will provide some protection.
Ambient air temperature sensor and RH sensors provide the benefit of obtaining measurement data for two parameters using a single sensor. The history and nature of these sensors, however, bring up issues that you should be aware of to ensure you are getting good measurement data. Selecting the sensor that will best meet your needs is also critical. I hope this article has provided you with helpful information for your measurement application.